1. Field of the Invention
The present invention relates in general to an oxygen sensing element and a process of manufacturing the same, and more particularly to an oxygen sensing element for accurately determining an oxygen concentration in exhaust gases emitted in particular by internal combustion engines and boilers, wherein the sensing element assures a high operating response at a relatively low temperature.
2. Discussion of the Prior Art
It is known to use zirconia ceramics or other oxygen-ion conductive solid electrolyte materials, for determining an oxygen concentration in measurement gases such as exhaust gases produced by internal combustion engines of motor vehicles, or boilers, according to the principle of an oxygen concentration cell. Based on the determined oxygen concentration, an air/fuel ratio of an air-fuel mixture supplied to the engines or boilers is controlled to assure an optimum combustion or burning condition of the engines or boilers.
An oxygen sensor or detector for determining an oxygen concentration as indicated above includes a sensing element which uses a suitably shaped tubular or planar solid electrolyte body made of an oxygen-ion conductive solid electrolyte material. On the inner and outer surfaces of the solid electrolyte body, there are formed a pair of electrodes, respectively. One of the electrodes which is formed on the inner surface communicates with an ambient air or atmosphere so that it functions as a reference electrode exposed to the ambient air, which serves as a reference gas having a reference oxygen concentration. The other electrode formed on the outer surface of the solid electrolyte body communicates with exhaust gases so that it functions as a measuring electrode exposed to the exhaust gases, which serves as a measurement gas to be measured. The sensing element is adapted to detect an electromotive force which is induced between the reference and measuring electrodes, due to a difference in the oxygen concentration between the atmospheres to which the two electrodes are exposed. Thus, the oxygen concentration in the exhaust gases is determined.
It is known that a .lambda. (lambda) curve representing a relationship between the output of an oxygen sensor of the type discussed above and the air/fuel ratio (A/F ratio) of an air-fuel mixture supplied to a gasoline engine, for example, ideally exhibits a sudden drop in the sensor output, at the stoichiometric A/F ratio, i.e., A/F=14.7 for a gasoline engine, as indicated in FIG. 1. This sudden change of the sensor output at the stoichiometric A/F ratio of 14.7 is utilized to control the A/F ratio of the air-fuel mixture.
In the known oxygen sensor, however, the A/F ratio at which the sensor output suddenly changes tends to be deviated or shifted away from the stoichiometric point on the side of higher A/F ratio values (to the right as viewed in the graph of FIG. 1) particularly when the temperature of the exhaust gases is relatively low, whereby the air/fuel ratio of the air-fuel mixture cannot be maintained at the stoichiometric point. This tendency has an adverse effect, from the standpoint of purification of the exhaust gases which result from the controlled air-fuel mixture.
To solve the above problem encountered when the temperature of the exhaust gases is low, there have been proposed various heater-built-in oxygen sensors which employ a heater for heating the oxygen sensing element, so that the sensing element is maintained at an optimum operating temperature even when the temperature of the exhaust gases is low. Certainly, such heater-built-in sensors are complicated in construction due to the incorporation of the heater within the sensor body, and are consequently less economical to manufacture.